Photosynthesis and Cellular Respiration: Photosynthesis

Transcription

1 Photosynthesis and Cellular Respiration: Photosynthesis

2 Unit Objective I can compare the processes of photosynthesis and cellular respiration in terms of energy flow, reactants, and products.

3 During this unit, we will answer the following very important questions:

4 #1: What is photosynthesis?

5 #2: How does the process of photosynthesis result in the storage of energy required by cells?

6 #3: What is cellular respiration?

7 #4: How are the processes of photosynthesis and cellular respiration related?

8 We will continue our study of this unit by focusing on photosynthesis. To do this, we will learn how to summarize how energy is captured from sunlight during the first stage of photosynthesis analyze the function of electron transport chains during the second stage of photosynthesis relate the Calvin cycle to carbon fixation during the third stage of photosynthesis identify environmental factors affecting the rate of photosynthesis

9 Even if you don t like eating your vegetables,

10 but would prefer to eat a hamburger instead,

11 EVERYTHING you eat, drink, or otherwise ingest contains energy (to keep you alive and properly functioning) that can be traced back to Earth s sun. AUTOTROPHIC HETEROTROPHIC

12 Without sunlight, there would be no life on Earth.

13 Ironically, only 1% of the energy in the sunlight that reaches Earth s surface is captured by plants, algae, and some bacteria, which convert the sunlight into chemical energy by means of photosynthesis.

14 Photosynthesis is the process that provides energy from sunlight for nearly all living things.

15 Photosynthesis occurs in three stages: --energy is captured from sunlight. --the light energy captured is converted into chemical energy, which energy is then temporarily stored in ATP and another molecule called NADPH --this stored chemical energy uses carbon dioxide (CO 2 ) to power the formation of organic compounds

16 Photosynthesis occurs in the chloroplasts of plant cells,

17 and in the cell membrane of certain prokaryotes.

18 In plants, leaves are the primary sites of photosynthesis because the cells of leaves contain numerous chloroplasts.

19 Plant leaves are thin enough to allow sunlight to penetrate into the cells, the chloroplasts, and the thylakoids.

20 Tiny openings in the leaf surfaces (called stomata) on the other hand allow carbon dioxide to enter the leaves, and oxygen and water vapor to leave.

21 The process of photosynthesis can be summarized in the following chemical equation: 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 carbon dioxide water sugars oxygen gas

22 Plants use the organic compounds they synthesize during photosynthesis to carry out their own life processes.

23 Some of these organic compounds are used to form starches, which plants store in their roots and stems.

24 Plants later break down these stored starches to power their own metabolism.

25 All of the proteins, nucleic acids, carbohydrates, lipids, and other molecules required for survival and proper functioning are assembled from fragments of these sugars, not just in plants,

26 but in most other living things as well.

27 Yellow and orange vegetables are rich in sources of carotenoids (also collectively referred to as beta carotene), important dietary sources of vitamin A.

28 Vitamin A is necessary for the development and maintenance of proper eyesight, for maintaining the health of cell membranes, and for proper tooth and bone development.

29 Vitamin A is also one of several molecules that acts as an antioxidant, a molecule that inhibits the oxidation of other molecules.

30 Oxidation is a chemical reaction capable of producing free radicals atoms, molecules, or ions capable of leading to chain reactions that may damage cells.

31 Fortunately, consuming food high in vitamin A and other antioxidants (vitamin C and compounds containing sulfur) interfere with the ability of free radicals to damage cells.

32 QUESTION Why do plants grown in the shade often produce larger leaves than plants grown in full sunlight?

33 Photosynthesis Stage One: Absorption of Light Energy The chemical reactions that occur during the first and second stages of photosynthesis are sometimes called the light reactions or light-dependent reactions.

34 This is because, without the absorption of light, these chemical reactions would not occur, and plants would not be able to produce the energystoring compounds they require for life.

35 Light is a form of electromagnetic radiation, waves of energy passing through physical space.

36 Different types of electromagnetic radiation have different wavelengths (the distance between two consecutive waves of energy).

37 Although Earth is bombarded by many different types of electromagnetic radiation (ranging from gamma rays to radio waves), the human eye is capable only of detecting that type of radiation known as visible light.

38 Humans detect visible light in different wavelengths as differently-colored light, ranging from indigo to red.

39 Sunlight contains all of the visible wavelengths of visible light.

40 Visible light, however, comprises only a small percentage of the total spectrum of electromagnetic waves traveling through physical space. There are other types of electromagnetic radiation which require no medium at all through which to travel gamma rays, X-rays, ultraviolet (UV) rays, infrared waves, microwaves, and radio waves.

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42 The human eye, the surface of plants, and all other objects and substances in the universe contain light-absorbing chemical substances called pigments.

43 Different pigments absorb only certain wavelengths of light, and reflect all others. Any wavelengths of visible light reflected by a pigment give an object the colors of the reflected wavelengths of light. Any wavelengths of light absorbed by a pigment go undetected by the human eye (because they have been absorbed). If, for example, an object reflects red wavelengths of light but absorbs all other wavelengths of visible light, it will appear red to the human eye.

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45 Chlorophyll, the primary pigment involved in photosynthesis, absorbs wavelengths of blue light and red light primarily, while reflecting wavelengths of green light and yellow light. This reflection of green and yellow light makes many plants especially their leaves appear green in color.

46 Plants contain two types of chlorophyll (chlorophyll a and chlorophyll b), both of which are instrumental to the process of photosynthesis.

47 Carotenoids, the pigments that produce the yellow and orange color in the autumn colors of leaves as well as the colors of many fruits, vegetables, and flowers, absorb wavelengths of light different than those absorbed by chlorophyll.

48 Having both pigments enables plants to absorb more light energy during photosynthesis.

49 The scientific study of the specific interactions between light energy and matter is called spectroscopy. A spectrophotometer is an instrument used to measure these interactions.

50 Many areas of biology are impacted by the study of spectroscopy, including the ability of specific pigments to absorb specific wavelengths of light energy as measured by a spectrophotometer. A graph plotting a pigment s light absorption as a function of wavelength is called an absorption spectrum.

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52 The pigments involved in photosynthesis in the chloroplasts of leaf cells.

53 Clusters of these pigments are embedded in the membranes of disk-shaped structures within the chloroplasts called thylakoids, which are the site of the light-dependent reactions of photosynthesis.

54 When an electron in an atom or molecule gains energy (from whatever source), the electron jumps to higher energy levels. Ultimately, the electron will fall to lower energy levels, releasing the energy absorbed in proportion to the magnitude of the drop in energy level. These emissions of energy may be either above or below the visible range of the electromagnetic spectrum.

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56 When light strikes a thylakoid in a chloroplast, energy is transferred to the electrons in chlorophyll molecules inside the thylakoid. This energy transfer causes the electrons to move to higher energy levels within the chlorophyll molecules. This constitutes the first step by plants to capture energy from sunlight.

57 Electrons energized by light energy jump from chlorophyll molecules to other nearby molecules in the thylakoid membrane, where the electrons full of energy are used to power the second stage of photosynthesis

58 These excited electrons that leave chlorophyll molecules, however, must be replaced by other electrons. Plants obtain these replacement electrons from water molecules.

59 Water molecules are split by enzymes within the thylakoid. The chlorophyll molecules in the thylakoid then obtain replacement electrons from the hydrogen atoms in the split water molecule, leaving hydrogen ions (H + ).

60 Oxygen atoms from disassembled water molecules then combine to form oxygen gas (O 2 ).

61 Phytoplankton, autotrophic components of Earth s vast plankton communities and a key component of Earth s marine and freshwater ecosystems, are among the most numerous organisms on the planet.

62 These tiny organisms float freely in Earth s freshwater and marine ecosystems. Combined, they are responsible for approximately 40% of the photosynthesis occurring on the planet.

63 Photosynthesis Stage Two: Conversion of Light Energy Energized electrons leaving chlorophyll molecules are used by chloroplasts to produce new molecules including ATP that temporarily store this chemical energy.

64 Excited electrons move from chlorophyll molecules to nearby molecules embedded in the membrane of the thylakoids of the chloroplasts.

65 The energized electron is then passed through a series of molecules along the thylakoid membrane,

66 similar to a ball being handed from person-toperson in a long line of people.

67 These series of molecules through which energized electrons are passed along the thylakoid membrane are called electron transport chains.

68 As energized electrons move through pass through proteins embedded in the thylakoid membrane, they lose some of their energy (because energy is required to move from one protein to another).

69 This lost energy is used by the thylakoid membrane proteins to pump hydrogen ions (H + ) produced by the splitting of water molecules into the thylakoid itself.

70 As photosynthesis continues, H + becomes more concentrated inside the thylakoid than outside, producing a concentration gradient across the thylakoid membrane.

71 As a result, H + tend to diffuse down their concentration gradients through specialized carrier proteins to the exterior of the thylakoids.

72 As H + move through these carrier proteins, the proteins catalyze chemical reactions in which phosphate groups are added to ADP to make ATP.

73 Thus, the movement of H + across the thylakoid membranes provides the energy required to synthesize ATP, which is used to power the thirs and final stage of photosynthesis.

74 While some electron transport chains synthesize ATP, other electron transport chains provide the energy to synthesize another molecule called NADPH.

75 Nicotinamide adenine dinucleotide phosphate (NADPH) is an electron carrier that provides the high-energy electrons required to bond carbon and hydrogen atoms during the third stage of photosynthesis.

76 NADPH is produced when energized electrons combine with H + and an electron acceptor called NADP +.

77 These electron transport chains tend to work in pairs one transport chain producing ATP while another produces NADPH.

78 During the first and second stages of photosynthesis, light energy is used to synthesize ATP and NADPH molecules, which temporarily store chemical energy.

79 These stages of photosynthesis are thus considered light-dependent.

80 During the third and final stage of photosynthesis, however, carbon atoms from carbon dioxide (CO 2 ) in the atmosphere are used to synthesize organic compounds in a process called carbon fixation.

81 These reactions that fix carbon are sometimes called dark reactions, or lightindependent reactions because these reactions can occur during either periods of sunlight or darkness (e.g., at night). Among photosynthetic organisms, there are several methods by which carbon can be fixed for photosynthesis.

82 Photosynthesis Stage Three: The Calvin Cycle The most common method by which carbon fixation occurs is the Calvin cycle.

83 The Calvin cycle is a series of enzyme-assisted chemical reactions that results in the production of a three-carbon sugar. The Calvin cycle occurs in the stroma (the colorless fluid surrounding the grana (collectively, the stacks of thylakoids within the chloroplast of plant cells) within the chloroplasts of a plant cell.

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85 Calvin Cycle, Step 1: Carbon Fixation During the carbon-fixation stage of the Calvin cycle, three molecules of CO 2 enter a plant and are added, one each, to three five-carbon compounds called ribulose 1, 5-biphosphate by an enzyme called ribulose-1,5-biphosphate carboxylase/oxygenase (better known as RuBisCo).

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87 Calvin Cycle, Step 2: Reduction The resulting six-carbon compounds split into six three-carbon compounds of 3- phosphoglycerate (also known as 3-PGA). Six phosphate groups from 6 ATP molecules (one for each 3-PGA) are added to these three-carbon compounds, forming six three-carbon compounds called 1, 3-biphosphoglycerate and 6 ADP molecules.

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89 Calvin Cycle, Step 2: Reduction Six electrons from 6 NADPH molecules (one for each 1, 3 biphosphoglycerate molecule) are then added to these 1, 3 biphosphoglycerate molecules to form six three-carbon sugars called glyceraldehyde 3-phosphate (more commonly referred to as G3P) and 6 NADP + ions.

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91 Calvin Cycle, Step 3: Carbohydrate Synthesis One of the six resulting G3P sugars is used to synthesize organic compounds including starch, sucrose, and cellulose.

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93 Calvin Cycle, Step 4: Regeneration The remaining G3P sugars are used to regenerate the initial ribulose 1, 5-biphosphate molecules that began the cycle, thereby completing the Calvin cycle.

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95 The Calvin cycle is named for American biochemist and Nobel laureate Melvin Ellis Calvin ( ), who discovered the chemical reactions in the cycle.

96 The energy required for the Calvin cycle is provided by the ATP and NADPH synthesized during the second stage of photosynthesis. The organic compounds produced during he Calvin cycle provide the organism with energy for growth and metabolism.

97 Photosynthesis is directly affected by various environmental factors, the most obvious of which is light.

98 In general, the rate of photosynthesis in plants will increase as light intensity increases until all of the pigments in a plant are being used and the Calvin cycle is operating as rapidly as it can.

99 The overall rate of photosynthesis, then, is limited by the progress of the slowest stage of photosynthesis, which is the Calvin cycle, the most time-intensive phase of photosynthesis.

100 The concentration of available CO2 also affects the rate of photosynthesis in a similar manner: the greater the concentration of CO2 (to a point), the greater the rate of photosynthesis.

101 Photosynthesis is almost most efficient within a certain range of temperatures. Like all metabolic reactions, photosynthesis involves many enzyme-assisted chemical reactions. Unfavorable temperatures may inactivate certain enzymes, decreasing the rate at which photosynthesis occurs.

102 Scientists the world over are studying the responses of plants to increasing atmospheric CO 2.

103 In general, it appears that increasing levels of CO 2 in Earth s atmosphere has resulted in an increase in biomass (specifically, plant tissue) during photosynthesis.

104 These plants, in producing more plant tissue, are countering the effects of rising atmospheric CO 2 levels caused by the combustion of fossil fuels by taking in more CO 2 from the atmosphere to make these tissues.

105 One unfortunate side effect of this phenomenon is that these new plant tissues appear to have less nutrients than previous plant tissues, which may effect the heterotrophs that feed on these plants.

106 A plant kept inside a home may receive 100 times less light than if it were grown outdoors. Some houseplants thrive in such dim light.